1. Technical Field
This invention relates generally to an ignition coil for use in producing a spark across a gap of a spark plug in an internal combustion engine, and more particularly, to an ignition coil having a primary winding disposed outwardly of a secondary winding.
2. Discussion of the Related Art
Known ignition coils utilize primary and secondary windings and a magnetic circuit. The magnetic circuit portion has taken a wide variety of configurations, and is typically constructed using magnetically-permeable material, such as steel laminations or compression molded insulated iron particles. One configuration known in the art comprises an elongated, generally cylindrical core.
This slender core is known to be used for so-called "pencil coils," useful in space limited engine configurations. In such a "pencil" coil, a number of components are typically concentrically arranged and include, from inside to outside, a core, a primary winding wound around the core, a secondary spool made of insulating material spaced from the primary winding, a secondary winding wound on the spool, a case formed of insulating material, and a shield formed of electrically conductive material and generally grounded. It is further known in the foregoing configuration to fill a first potting channel defined between the primary winding and an inside diameter of the secondary winding spool with epoxy potting material (i.e., dielectric material), and further, to fill a second potting channel defined between the secondary winding and an inside diameter of the case with epoxy potting material. The potting process for this configuration therefore involves filling two potting channels with potting material.
Another known configuration includes the following components, also generally concentrically arranged, in order from inside to outside, a core, a secondary winding spool made of insulating material, a secondary winding wound on the secondary winding spool, a primary winding spool, a primary winding wound on the primary winding spool, a case made from electrical insulating material, and a shield (grounded). This known configuration also includes two potting channels, namely, the annular space between the secondary winding and an inside diameter of the primary winding spool, and, the annular space between the primary winding and an inside diameter of the case. The potting process for this configuration thus also involves the filling of two potting channels. In addition, three principal dielectric barriers exist between the shield and the core, namely, the case, the wall of the primary winding spool, and the wall of the secondary winding spool (in addition to the epoxy potting material which makes some dielectric contribution). The increased capacitance resulting from the dielectric barriers impacts the secondary voltage performance (i.e., the voltage characteristics delivered to the spark plug). Since the form factor of the above-described configurations (i.e., slender form factor) is useful, it would be desirable to improve upon the foregoing.
Thus, there is a need to provide an improved ignition coil that minimizes or eliminates one or more of the shortcomings as set forth above.